Instituto de Investigaciones en Catálisis y Petroquímica - INCAPE (FIQ, UNL-CONICET)

Santiago del Estero 2829 – 3000 - Santa Fe - Argentina.Phone/Fax:54-342-4571162.e-mail:nfisico@fiqus.unl.edu.ar

The (SCR) of NO_x to N₂ is an important commercial procedure to remove pollutants from exhaust fumes. For stationary sources the SCR of NO_x is achieved using NH₃ as a reducing agent . Despite the general use of such a process, there are problems associated with NH₃ as reducing agent which include ammonia slip, equipment corrosion, etc.

The use of CH₄ as reducing agent has become particularly interesting, pioneered by the work of Li and Armor . They found that CoZSM5 and CoFerrierite appear to be more active than other transition metal ions for the SCR of NO_x with CH_4.Since then, a number of active catalysts for such reaction have been reported, i.e., Ga and In-loaded HZSM5, noble-metal based catalysts exchanged in ZSM5 , particularly PdHZSM5 , and Pd-based catalysts supported on different solid acids and zeolites. However, none of these catalysts are sufficiently effective in wet atmospheres; consequently, substantial improvement is still needed for this process to be economically competitive with the NH₃ SCR process.

One of the avenues that many researchers have explored to achieve the above objective is to create catalysts of high activity and selectivity by the combination of catalytic sites for NO oxidation and NO reduction .

In this contribution the selective catalytic reduction of NO_x with CH₄ in excess oxygen was studied over a series of Zeolites (viz. Mordenite, ZSM5, Ferrerite and Y Zeolite) containing either Co or Pt, or both.

By Pt and Co ionic exchange, catalysts were prepared on the following zeolitic matrixes: Na-mordenite [Na₇(AlO₂)₇(SiO₂)₄₁], Na-ZSM-5 [Na₉(AlO₂)₉(SiO₂)₈, K-Ferrierite [(Na,K)_3,7(AlO₂)_3,7(SiO₂)_32,3] and Na-Y[Na₅₆(AlO₂)₅₆(SiO₂)₁₃₆].

The alumina-based samples were prepared by the wet impregnation method using Pt(NH₃)₄(NO₃)₂ for Pt/Al₂O₃ and Co(NO₃)₂ for Co/Al₂O₃. The Pt/Co/Al₂O₃ catalyst was prepared by successive impregnation.

To ascertain the nature of the Pt promoting effect, mechanical mixtures and different combinations of the two-bed system made up of the monometallic zeolites were also investigated.

To substantiate the role of the zeolite framework, mono and bimetallic alumina supported catalysts were tested. In all cases, the catalytic tests were run with and without water added to the feed stream. XPS studies, FTIR and Temperature-programmed reduction of the different catalyst systems provided information concerning the nature of species presents in the solids and the Co-Pt interaction.

A promotion effect was observed for the NO reduction to N₂ by CH₄ when Pt and Co exchanged in zeolites were combined, Pt also strongly enhanced the water resistance of Co-zeolites. Such effect was a function of the degree of interaction of the active species Pt_0.5Co_2.0Mordenite > PtMor + CoMor (mechanical mixture) @ PtMor/CoMor (Two-stage catalyst) > Co_2.0Mor. Pt_0.5Mor was inactive under the reaction conditions used.

The intrinsic activity of Pt promoting catalysts depends on the support and the zeolite matrix. The order of activity (under wet reaction stream) expressed as turnover frequency was Pt_0.5Co_2.0Ferr > Pt_0.5Co_2.0Mor > Pt_0.5Co_2.0ZSM-5 >> Pt/CoAl₂O₃.

XPS and TPR results show that the species the reactant sees are Pt°, Pt²⁺, Co²⁺, Co° and some H⁺ generated during reduction. Such species remain under dry and wet reaction conditions but some reorganization occurs .

The highest activity and selectivity of the PtCo-zeolite could be explained as a synergetic effect (cooperative effect) of the active species (Pt° and exchanged Pt²⁺, Co²⁺ and H⁺) which increased the activity for the NO to N₂ reaction and the activation of CH₄. It also enhances the ability of the catalyst to adsorb NO. These hypoteses are discussed in terms of what has been reported in the literature for comparable systems.